1. Field of the Invention
The present invention relates to a molded optical element, such as a lens, mirror or the like, and more particularly, to an improved plastic molded optical element for facilitating optical surface inspection on a mass production basis.
2. Description of the Prior Art
The art of plastic injection molding of optical elements such as camera lenses has advanced and become more accepted in higher quality optical systems. This has resulted from an increase in labor costs and the fact that both the molding art and plastic material utilized have become more dependable on a mass production basis. The use of plastic lenses, for example in photography, has further provided optical designers with an economical method of providing aspherical surfaces for controlling various aberrations in a lens system.
The traditional method of grinding and polishing glass lens elements provided a high degree of reliability although it is relatively time consuming and expensive from a labor viewpoint. When a glass lens element is finished, there is a relatively high confidence level that the optical surface of the lens element is free of any defects.
The molding of plastic lens elements has lessened both the time and the labor required; however, the same level of confidence of optical accuracy is not as easily obtained. The quality of plastic material, the existence of gaseous bubbles and even the ambient temperature and humidity can affect the surface quality of the resulting molded optical element.
Frequently, the prior art has utilized a form measuring instrument to scan the optical surface along a given line and measured the relative height of that surface at each point by an electrical comparator. Any resulting measured errors are appropriately noted and if beyond the tolerance requirements would result in rejecting that particular optical element. In making these surface measurments, the scan path is usually traced along a line running through the center of the optical surface to be inspected. It has, however, been very difficult to determine whether the scanning is truly made along the desired line. This is particularly true in a mass production environment. Usually the scanning path or line was determined in accordance with an operator's experience or his reliance upon the output of a form measuring instrument based on the assumption that the output of this instrument would disclose either a maximum or minimum value at the center of the surface being measured. The reliance upon an operator's experience, however, adds an undesirable subjective possibility of error while the other method is inherently faulty in that it cannot indicate the center if the optical element is not properly aligned, such as when the surface to be measured is inclined relative to the measuring instrument. As a result, the prior art is still attempting to improve quality control on the inspection of molded plastic optical elements for mass production.
Cited of general interest is U.S. Pat. No. 4,015,897 which discloses plastic molded lenses for use on a camera and more particularly discloses a pair of meniscus lenses with molded integral peripheral flanges for mounting and spacing the lenses in the lens barrel.